Gas composition tracking in transient pipeline flow

Tracking changes in gas composition in natural gas pipeline transport systems is becoming increasingly important to pipeline operators, since gas suppliers are shifting from long-term delivery contracts to shorter-term contracts, increasing delivery of gases from unconventional sources, and proposing to inject hydrogen and biomethane into the natural gas networks. The present study investigates two methods for tracking gas composition, one using a moving grid method, and one solving the advection equation using an implicit backward difference method. The methods were applied to a model of an onshore pipeline in the Polish transmission system, and a model of an offshore pipeline in the Norwegian transmission system. The differences between the measured and modeled compositions and transport times were investigated. Both methods reproduced the measured compositions and transport times well, with an error in total transport times of less than 2.0%. The implicit method was found to lose some of the finer details of the gas composition profiles due to numerical diffusion, while the moving grid method preserved the composition profiles.